Transmission/reception circuits for digital communication are frequently implemented using the direct mixer principle. In this case, the complex-value baseband signal comprising an inphase component and a quadrature component is converted directly to the RF transmission frequency in the transmission path. In the reception path, a received RF signal is downconverted directly to the center frequency 0, is split into the inphase and quadrature components and is processed further.
The great demands on the signal quality of high-quality modulation types such as 16 or 64 QAM (Quadrature Amplitude Modulation), which are demanded for WLAN (Wireless LAN), for example, mean that nonlinear characteristics for the RF components, particularly the mixers, amplifiers and the filter stages in the transmission and reception paths, can have a particularly disruptive effect. The nonlinearities appear in the form of an amplitude or phase error, which is generally referred to as an IQ mismatch. The IQ mismatch manifests itself at the transmission end and the reception end by impairing the signal-to-noise ratio or by increasing a vector error at the IQ level, this being referred to as the “Error Vector Magnitude (EVM)”.
An additional, unwanted DC component, the DC offset, produces a signal in the transmission path at the carrier frequency, which is usually situated centrally in the useful signal range. A DC component in the reception path likewise results in a signal close to the center frequency 0.
During production, either calibration to correct the sources of error is performed directly on the RF components or digital compensating circuits, referred to as predistorters, are used which compensate for any amplitude or phase error which arises and for a DC component by adding an additional signal to the useful signal or altering the useful signal. Fluctuations in the errors during the product lifecycle as a result of aging and during ongoing operation as a result of changes in the temperature are not compensated for by fixed presets, however.
The inventor is aware of a method for compensating for an IQ mismatch and a DC component in the transmission path of a transmission and reception stage. In this case, the powers of the parasitic signals of the IQ mismatch and of the DC component are gauged in the transmission path. This can be done by determining distortions in the output signal from the transmission path, for example. A transmission arrangement which is suitable for this purpose is known from the printed document U.S. Pat. No. 5,396,190. This document describes the practice of demodulating the distorted output signal again and ascertaining correction coefficients therefrom in a suitable fashion.
Printed document U.S. Pat. No. 6,147,553 shows a transmission path having a plurality of amplifier chains which are arranged in parallel and which each have compensating circuits connected upstream of them in order to perform amplitude and phase distortion for each amplifier chain. Errors which arise in the reception path of a transceiver, however, are not eliminated thereby.